A: From our SAE paper 970947 SMAC-97:
- "The Inter-vehicle friction coefficient of the SMAC collision model (AMU, SMAC input card 13, field 7) is based on coulomb friction. Coulomb friction is independent of the sliding velocity. Coulomb friction should have a coefficient value of less than or equal to 1.0. During the collision, as relative sliding of the surfaces occurs, the magnitude of the inter vehicle friction force depends on the existence and magnitude of the collision normal force. Supplemental tensile forces and/or moment constraints sometimes occur in vehicles as the vehicles begin the separation phase when the normal force may be very small or zero. After the primary collision, as a result of the impact configuration, the vehicle contact area and/or the vehicle component contacted, intermeshed components of the two vehicles can offer additional resistance to separation in the absence of the normal forces required for coulomb friction. Using a value for the inter-vehicle friction coefficient greater than the recommended range (normal value 0.55, recommended range 0.3-1.0) may unrealistically and/or adversely affect the primary impact phase, since the additional forces and moments produced by the elevated friction can act to redirect the forces and moments during the primary impact."
The appendix includes an attempt to justify using intervehicle friction >> 1.0.
- Because in the RICSAC validation studies of SMAC “RICSAC – Vol IV – Staged Collision Reconstructions” Jones & Baum used in some instances values for inter-vehicle friction greater than 1.0.
- The same input values for inter-vehicle friction > 1.0 were carried forward in the validation work of EDSMAC by Day & Hargens SAE 900102, Further Validation of EDSMAC using the RICSAC Staged Collisions.
- ALL values for inter-vehicle friction coefficient were < 1.0
- (See below for the actual values used in the 2003 SMAC validations).
- Paper "2003-01-0486" validated the SMAC program by producing results for impact speeds and deltaV all less then 10% of the measured values while starting with CRASH/linear momentum results and then automatically iterating to a 'match' of the evidence (damage and scene). (NO human intervention in the iteration process and the initial starting values from CRASH which were sometimes >>20% in error). Marine, et al never contacted us about what values we used in our 2003 paper (and as we mentioned all were less than 1.0).
- "A secondary problem which may occur with original SMAC (EDSMAC) use of a high value of inter vehicle friction can occur in the post-processing program which may not be able to "match" the accelerations to the damage region. As a result invalid Delta-V values and clock direction may be reported. An EDSMAC application summary page (used by an expert witness as an exhibit to deposition testimony) which contains invalid Delta-V values for the collision partners was produced by the use of a value for the inter vehicle friction coefficient of 2.0 and is shown in in the attached Figure. (Note that in the EDSMAC application m1*DeltaV1 does not = m2*DeltaV2). SMAC creates a supplementary diagnostic page which reports all of the impact speed changes calculated for a given run to permit a check of the values reported by the post-processing routines. This auxiliary page appears to have been omitted in the EDSMAC program"
- Any need for the use of a value of intervehicle friction > 1.0 indicates that the force of collision is not a function of the normal force and most likely a snag or inter-locking of the vehicle occurred and therefore that the SNAG option of SMAC should be used.
- Also in the SMAC Input Manual (Help->SMAC Input Manual) OR also online in the SMAC input Manual DISUCSSION: SNAG Option